1. Field of the Invention
The present invention relates to a flow element that measures a flow rate or a flow volume of a subject fluid based on the heat transfer phenomenon from a heating element or a part heated by a heating element and is used for measurement of, for example, an intake air flow of an internal combustion engine.
2. Description of the Related Art
A flow element utilizing a temperature dependent resistive element has conventionally been known. Such a flow element is equipped with a heating element and an intake air temperature detection element, and a sensor device controlled in such a way that the temperature of the heating element stays at a higher temperature by a certain temperature range than the temperature detected by an intake air temperature detection element is utilized to provide an output in the form of a voltage corresponding to a heat quantity the heating element radiates to the fluid.
As is described in Patent Document 1 (Japanese Patent Laid Open Hei 9-26343), for example, a flow element comprises of a cavity formed at the backside of a planar semiconductor material by partially removing the backside of the planar semiconductor material and a thin film having a detection element just over the cavity. Only one side of the sensor device is adhered to an engaging portion of a support member to form a floating support structure (cantilever support structure).
The flow element with a floating support structure, as is described in Patent Document 1, has such a problem that in a small flow volume of the subject fluid, it flows only over the surface of the sensor device, while, in a large flow volume of the subject fluid, it flows into a clearance gap between the sensor device and the engaging portion of the support member (hereinafter referred to as “underflow”), which leads to deterioration of flow detection accuracy. In order to solve such a problem, Patent Document 1 sets a canaliform slot within the engaging portion of the support member alongside the periphery of the sensor device so as to prevent the underflow from contacting directly to the sensor device.
However, such a structure cannot prevent the underflow sufficiently, because such a case happens that the underflow led into the slot overflows into a clearance gap between the sensor device and the engaging portion of the support member. One possible method to prevent the underflow of the flow element with a floating support structure is to use an underflow inhibitor which is a type of adhesive cement.
One such method, for example, is to apply first an underflow inhibitor 3 to the side surface of an engaging portion of a support member 2 as is shown in FIG. 17 (a), and then to engage a sensor device 1 into the engaging portion 2 as is shown in FIG. 17 (b), so that the underflow inhibitor 3 is filled up into the gap between the sensor device 1 and the engaging portion 2, which prevents the generation of the underflow.
However, in the flow element utilizing the underflow inhibitor 3, when the sensor device 1 is engaged to the engaging portion 2, occurrence of application quantity fluctuations of the underflow inhibitor 3 applied to the side surface of the engaging portion 2 could lead to the underflow inhibitor 3 sticking out of the front surface of the sensor device 1 and a cavity 4 of the backside of the sensor device 1 as a result of flow of the underflow inhibitor 3 in the direction of an arrow 18 as shown in FIG. 17 (c).
When the underflow inhibitor 3 sticks out to the front surface of the sensor device 1, turbulence of a flow 15 of the subject fluid at the front surface of the sensor device 1 is caused, and fluctuation of the flow element output occurs. Also, when the underflow inhibitor 3 sticks out into the cavity 4 of the backside of the sensor device 1, the stuck out underflow inhibitor 3 adheres to a thin film 23, which faces the risk of damage.